Paleo Slide Set: The Ice Ages
Portrait of Milutin Milankovitch by Paja Jovanovic, 1943, courtesy of Vasko Milankovitch

Croll's arguments electrified the scientific community and provoked a great deal of debate and research. 
Geologists scoured Europe and the Americas in search of evidence to support or refute Croll's astronomical 
theory. Initially, the theory seemed to square well with the geologic evidence, but by the 1890s terrestrial 
field research raised serious questions about its accuracy. Croll's calculations predicted that the end 
of the last glacial period occurred some 80,000 years ago and yet reasonable estimates placed the age 
of two North American waterfalls that had formed after the great ice sheets retreated at 6,000-32,000 years 
(Niagara on the U.S.-Canada border) and 6,000-10,000 years (the Falls of St. Anthony in Minnesota). 
Clearly, something was wrong with Croll's theory. At the dawn of the twentieth century, few reasonable 
persons believed that variations in eccentricity and precession caused the ice ages.

However, in the 1910s, the Serbian mathematician Milutin Milankovitch, began to embark on a series 
of calculations that would eventually revive the orbital theory of climate change. Milankovitch's 
main contributions were threefold. First, he used new astronomical calculations by the German scientist 
Ludwig Pilgrim that took into account a third cyclical variation in the earth's orbit, that of obliquity 
or tilt. Secondly, he reasoned that summer rather than winter temperatures were the main contributors 
of ice sheet growth and decline. Lastly, he calculated summer radiation curves for the key latitudes 
of 55, 60, and 65 degrees N that seemed to correlate well with evidence then available from the geologic record.

Photo Credits:
Contributed by John Imbrie
Brown University